When you get some time look way back in the datnoid threads and you will come to a few discussions on NTT sudden death syndrome.The info there may shed some light on your situation.
Yes, there are many threads reporting SDS in NTT, but for no obvious reason.
I know the water conditions well. Virtually all NTTs come from the Mehkong River as it passes the North East of Thailand (not the North). It is a massive river, varying much in height and flow. Freshwater - absolutely not brackish. I have been up and down this stretch of water many times, and have heard of some large NTT being caught. However, in the trade, I have never seen one bigger than four inches. The two that died, and the one I have left, I grew out to about eight inches.
I used to keep ST - real ST at that time. It was easy to grow them out to fifteen inches, and then they would start to fatten out a lot. But they were quite hardy. Currently I have six IT and they are similar, some currently about nine inches. I have got a NGT and an AT as well. All doing well.
I thought I was doing well with the NTT too. I will try and get some more.